Mutation

"Mutation is a type of Persona growth that happens sometimes after Fusion Spells. The better the compatibility, the likelier they are. How the Persona grows depends on its rank. The higher its rank at the time of mutation, the likelier a rare or useful mutation is. Some can become a different Persona entirely by selecting "Mutation" in the Velvet Room. A few Personas can only be obtained this way."

—Persona 2: Innocent Sin

Mutation is a gameplay mechanic in the Persona 2 duology. It differs from Evolution in that the Persona does not necessarily change forms, rather the Persona gains power or a new skill, although it is also possible for a Mutation to allow the Persona to evolve.

Persona can Evolve into a new Persona, usually a Persona of the Minor Arcana, however a few can "Rank Up", evolving to a higher-level persona of the same Major Arcana. Persona must be taken to the Velvet Room to change forms and returns to Rank 1. "Rank up" mutations can occur if the result is no more than 10 levels above the protagonist, however Minor Arcana mutations do not have this limit.

Persona gains its Unknown Power, an ability similar in concept to "desperation attacks" - powerful abilities that activate at random, and can only activate if the user is at critical health. There are 6 different types of Unknown Powers; each Persona has one specific type of Unknown Power associated with it.